In integrated circuit technology, it is often required to make or break electrical connections in order to fabricate and/or modify a circuit. For example, in programming logic arrays, certain gates and circuit elements must be connected while others must be isolated. Also, individual memory cells of programmable read only memories can be programmed by selectively connecting and isolating individual cells from certain conducting lines.
One common form of programming structure used to selectively program links between conductors is known as a programmable link structure or antifuse. The typical antifuse structure includes a lower conductor separated from an upper conductor by a transformable link insulator. Before programming, the upper and lower conductors are electrically isolated from each other. After programming, a permanent conductive link is formed between the conductors across the transformable insulator such that the upper and lower conductors are permanently connected to each other.
Individual programmable links are typically programmed by the selective application of power at the link structure. For example, prior antifuses were voltage programmable links (VPLs) programmed by applying a voltage across the upper and lower conductors. The electric field across the transformable insulator causes it to break down. The resulting current flow through the insulator heats the region around the link causing the conductors to melt and migrate into the insulator. As a result, a permanent link across the transformable insulator remains.
Another type of interlevel link is programmed by selective application of laser radiation at the programmable link structure. Laser light striking the upper conductor causes the area around the link structure to heat. The heat simultaneously causes the transformable insulator to break down and causes the metal conductors to migrate into the insulator. When the laser radiation is removed, the link site cools, leaving a permanent conductive link across the transformable insulator between the upper and lower conductors.
In antifuse devices, the breakdown characteristics of the transformable insulator determine the amount of power required for programming. One important factor which affects the breakdown characteristics of the insulator is its thickness. In general, the thinner the insulator, the lower the applied voltage required. Thus, for a VPL, a thinner insulator will generally require a lower programming voltage.
In integrated circuits, it is desirable to fabricate programmable link structures which require relatively low programming voltages to prevent stressing other devices in the circuits during programming. This results in link structures having very thin link insulator layers. However, with such thin insulator layers, unprogrammed link structures exhibit relatively high capacitance, resulting in slower overall circuit operation. In addition, such thin insulator layers can contribute to decreased lifetime of the circuit due to inadvertent programming at circuit operational voltage levels after a relatively short period of time.